1. Field of the Invention
The present invention relates to an anti-icing system for an aircraft.
2. Background Art
The anti-icing system most widely used for airfoil leading edges and engine inlets on commercial airplanes utilizes engine compressor bleed air. The system is an on/off type system that is sized by a low power setting idle descent icing condition. The system is oversized for most other conditions and is therefore wasteful. Also, a great portion of the energy bled from the engine is wasted before it reaches its destination. There are bleed port pressure losses, duct pressure losses, temperature reduction due to the pre-cooler under certain conditions, temperature losses in the ducting and the temperature and pressure loss in the spray hole nozzles. Commonly, after the warm air performs its deicing or anti-icing function against the wing leading edge, this air is dumped overboard, and this is a major energy loss.
Another disadvantage of the prior art system is that bleed air extraction from the engine causes thrust loss and an increase in specific fuel consumption. With high by-pass engines becoming more efficient, the engine gas generators becomes even smaller so that the bleed air penalties are larger yet. For these and other reasons, various alternative anti-icing systems have been investigated.
One consideration is to use a spray system where a substance such as glycol is sprayed onto the wing surface. However, this has certain drawbacks and then it would require use of a large amount of the deicing material. Also there is the problem of contamination of the wing surface.
Another prior art approach is to use inflatable rubber boots to form the leading edge of the wing. While this approach was widely used for deicing slower propeller airplanes, it generally does not provide a smooth enough surface for a high performance airplane. Also, the boot needs fairly frequent replacement due errosion and aging.
Another approach is to position electric resistance heaters, such as heater blankets, against the inside of the leading edge skin. The problem with this arrangement is the poor conductivity of the thin-air layer between the blanket and the leading edge skin. To heat the wing skin to a safe temperature, the blanket temperature has to be very high.
A search of the patent literature has disclosed a number of deicing systems. These are as follows:
U.S. Pat. No. 1,868,468, Thompson, discloses the use of electric heating elements located in the leading edges of the wings and other aerodynamic components of an airplane to alleviate the problem of these surfaces possibly becoming coated with ice. A rheostat controls the amount of current to the heating elements, and heat from these elements is applied directly to the leading edges.
U.S. Pat. No. 1,943,062, Driscoll, discloses an anti-icing system where a plurality of electrically conductive wires are positioned at spaced locations along the wing surfaces. When moisture, in the form of sleet or ice, begins to form on the wings, electric current will flow from one wire through the sleet or snow to the other wire, so as to melt the sleet, ice or snow and cause it to be removed from the wing.
U.S. Pat. No. 2,304,686, Gregg, shows a deicing system for an aircraft where a coil is placed about the exhaust stack of the aircraft engine so as to absorb heat therefrom. The heat exchange fluid in the coil is then directed through a manifold which is in proximity with the aircraft surfaces which are to be deiced. In another arrangement, there are electric heating coils mounted to the forward portion of the engine cowl. A commutator may be used to cause the flow of current or interrupt the flow of current, as needed, for deicing.
U.S. Pat. No. 2,390,093, Garrison, shows a deicing system for an aircraft where the leading edge surface of the wing is made of a porous material (i.e. porous sintered metal plates) through which anti-icing fluid is directed.
U.S. Pat. No. 2,418,205, Taylor, shows a deicing system where the leading edge of the wing is an inflatable structure which can be distorted to break up the ice which forms on the leading edge. The airfoil surfaces immediately rearwardly of the leading edge are stationary surfaces and are heated by other means. One system places heat exchange tubes adjacent the upper and lower surfaces, and a heat exchange medium is directed through these tubes to provide heat and then through return tubes, so as to form a closed loop system. Also electrically conductive heating elements are used to heat these upper and lower surfaces.
U.S. Pat. No. 2,591,757, Young, shows an airfoil where the front spar can be made in sections, and a heating element is provided at the juncture point of the two sections forming the forward part of the spar.
U.S. Pat. No. 2,869,535, Horrell, shows a deicing system where ambient air is drawn into a duct and moved by a fan through a heater, with the air passing from the heater being directed through a passageway at the leading edge of the wing.
German patent No. 709,354, shows what appear to be heating tubes disposed in the leading edge of an airfoil.